Software referred to as an LTFS (Linear Tape File System) allowing access to a tape medium via an interface of a typical file system has been provided. The LTFS has open specifications, and is achieved by storing data on a tape medium in a format according to an LTFS format. The LTFS is implemented by multiple vendors, such as International Business Machines™ Corporation, Oracle™ Corporation, Hewlett-Packard™ Company, and Quantum™ Corporation.
Use of the LTFS allows applications having previously used hard disk drives to access tape media without any change. Furthermore, a hierarchical storage has been configured where the LTFS is hierarchized as a secondary storage tier for a high-speed storage tier, such as HDDs or SSDs, without directly accessing files on the LTFS. This configuration can prevent the applications from timeout due to a required time period for accessing the files on tape media that is longer than an assumed time.
In the hierarchical storage, a high-speed storage is typically configured as a primary storage tier, and the applications store files in the high-speed storage. At a specific timing, the files are migrated to tape media on a secondary storage tier. After the migration, files that are referred to as stub files and indicate the presence of the files are stored on the primary storage tier. In the case where a file is subbed, an access from an application to the file causes the file to be read from the secondary storage tier and migrated to the primary storage tier, and the access is thus responded.
As to such a stub scheme, for example, JP2010-536079A (Patent Literature 1) has been known. Patent Literature 1 discloses a configuration in which an inode for a migration target file is held in an online storage as a stub file which is viewed as an original file from user applications in order to secure transparency of HSM operations.
As described above, stubbed files having no entity on the primary storage tier are read from tape media on the secondary storage tier. However, the tape media typically span at least 800 m in the longitudinal direction. Accordingly, the time required for locating varies according to the storage positions on the tape media where files are recorded. This variation classifies files into files that require a short response time for an access, and files that require a long response time. Therefore, a technique capable of managing a file space so as to minimize the number of files having long response times in a hierarchical storage as much as possible has been desired to be developed.